A small mismatch in engine speeds on a multi-engined propeller aircraft can cause undesirable acoustic noise and vibrations corresponding to the difference in engine speeds. One method of controlling noise and vibration--synchrophasing--involves comparing the rotational speeds and phase relationships of the aircraft engines. The synchrophasing system, using one engine as a master engine, adjusts the engine speeds of the remaining "slave" engines to be equal to the speed of the master engine. The engine speeds of the slave engines are further trimmed so that any given blade or propeller of a slave engine is in a fixed phase relationship relative to a corresponding blade or propeller of the master engine.
In the case of multi-spool engines, such as turbofan, turboprop or prop-fan engines, only one spool is synchrophased. The spool that is synchrophased is normally the fan spool on a turbofan engine or the prop spool on a turboprop or prop-fan engine. However, undesirable noise or vibration may still be produced by the non-synchrophased spools.